1. Field of the Invention
The present invention relates to a method of intracranial pressure measurement, and more particularly, to a non-invasive method of using an ultrasound contrast agent and a specific signal process for measuring intracranial pressure.
2. Description of the Prior Art
Regarding traumatic intracranial hematoma, intracranial tumor, hemorrhagic cerebrovascular disease, meningitis, or congenital cranial bone malformation, when a patient suffers from an attack of one of such diseases, intracranial pressure is usually increased. Due to meninges, blood vessels, or nerves being pressed, the patient might experience continual headaches and vomiting. What is worse, the patient might lose his vision because optic nerves can become atrophied due to optic papilla oedema. Therefore, if high intracranial pressure can be detected earlier and treatments are immediately executed, these problems can be alleviated.
Generally, references for detecting whether intracranial pressure is increased are clinical symptoms, such as headaches and vomiting. However, precise detection should be a main method for determination. There are three main detection methods as known in the prior art. One is to analyze cerebrospinal fluid extracted by lumbar puncture; another is to take an X-ray and inspect a gyri-pressure graph, bone symphysis, thickness reduction of cranium, and expansion of sella turcica, etc.; and the last is brain ultrasonic examination.
Lumbar puncture is an invasive method that has problems of infection and patient adaptation. X-ray and inspection of such are non-invasive methods, but are not efficient ways for early detection of high intracranial pressure. Ultrasonic signals used in ultrasonic examination are dramatically attenuated after traveling through the cranium, and thereby echoed signals are weak.
In recent years, in order to improve the quality of ultrasonic signals, an injection of contrast agent into blood or lymph has been used. Micro-bubbles of such a contrast agent are helpful in creating better acoustic wave feedback. Therefore, the purpose of signal improvement is achieved, which assists in measuring related parameters.
Please refer to FIG. 1, which is a frequency spectrum of ultrasound echoed signals associated with the contrast agent. As shown in FIG. 1, there are a fundamental response 11, a second harmonic response 12, and a subharmonic response 13. The latter two are non-linear and require higher emitting sound pressure to generate micro-bubbles, wherein the sound pressure required by the subharmonic response 13 is the highest.
The fundamental response 11 can be found in blood-flow and peripheral tissue, and thereby the fundamental response 11 cannot be used for comparison and recognition.
For one thing, after the second harmonic response 12 travels through the cranium, the second harmonic response 12 is dramatically attenuated due to its high frequency. Additionally, the second harmonic response 12 also occurs in mammal tissues. So it is difficult to use the second harmonic response 12 to distinguish between blood, lymph, and peripheral tissue.
A way for detecting the subharmonic response 13 is disclosed in U.S. Pat. No. 6,302,845. The patent uses a conventional ultrasound system assisted with contrast agent to estimate the pressure of the heart or portal vein. When micro-bubbles are under different pressures, differences of subharmonic responses are used for calculating the pressure accordingly. However, when the obvious subharmonic response 13 is excited by high pressure micro-bubbles can break. If the method is used for measuring intracranial pressure, micro-bubbles breaking might be a threat to the brain.